With $10,000 at 7% compounded monthly and $100 monthly contributions, your total grows to approximately $59,000 after 15 years. The initial $10,000 alone would grow to about $28,300, while the monthly contributions and their compounded interest add roughly $30,700. This demonstrates the power of combining a solid starting amount with regular contributions.

A 7% annual return is often cited as a reasonable long-term expectation for a diversified stock market portfolio, roughly matching the S&P 500's historical average after adjusting for inflation. Actual returns vary significantly year to year and depend on your asset allocation and market conditions. Conservative investors may want to model 5-6%, while aggressive growth portfolios historically averaged 8-10%.

Statistically, lump-sum investing outperforms dollar-cost averaging about two-thirds of the time because markets tend to rise over time. However, investing gradually reduces your risk of investing everything at a market peak. If you have $10,000 ready and a long time horizon of 15+ years, lump-sum investing is generally the better mathematical choice.